The catalytic cracking reaction of petroleum hydrocarbons is a secondary processing of crude oil, and mainly consists of gaseous- and solid-phase cracking, wherein its feedstock generally has a specific gravity greater than 0.9. The reaction process generally produces coke in an amount of 5.5% to 10.0% of the feedstock, and the coke adheres onto the surface of the catalyst and blocks the micro-pores in the catalyst, which necessitates recycle and regeneration of the catalyst.
The catalyst regeneration process plays an important role in the catalytic cracking reaction, with respect to equipment investment, operational energy consumption, and maintenance cost. Reducing the excessive oxygen in flue gas, reducing the gas pressure drop in the regeneration system, reducing the catalyst inventory, and reducing hydrothermal deactivation of catalyst caused by hydrogen reaction, all have great significance for improvement in the regeneration effect and the catalytic cracking process.
Typical catalyst regeneration techniques currently available include two-stage regeneration, the countercurrent two-stage regeneration techniques. In particular, the countercurrent two-stage regeneration technique developed by UOP and the Beijing Design Institute of Sinopec exhibits high efficiency. U.S. Pat. No. 4,299,687 discloses a reaction-regeneration technique based on two-stage countercurrent regeneration, co-developed by UOP and Ashland. CN 97121795.5 discloses a stacked two-stage regeneration technique for fluid catalytic conversion of heavy oil.
The catalyst is a key element to the catalytic cracking reaction of petroleum hydrocarbons, but its performance during the reaction is different from that before the reaction. For example in the case of catalytic cracking reaction of hydrocarbons, the catalyst performance during the reaction may gradually goes down because of coke formation, contamination by alkaline components and metals, and passivation during the reaction. In addition, reaction conditions such as the catalyst activity, the catalyst-to-oil ratio or the space velocity, and the temperature and temperature difference of feedstock and catalyst in the reaction are all important factors influencing the reaction results, and changes and control of these conditions would change the reaction results.
The target product of catalytic cracking reaction of petroleum is the component having a carbon number higher than C3, while the small-molecule products of C2 or lower are mainly produced by thermal cracking. Enhancing the catalytic cracking reaction while reducing the thermal cracking has always been a pursued objective for the catalytic cracking process. During catalytic cracking reaction, the selection and control of reaction conditions are directly related to the conversion rate and product selectivity of the reaction. There are many techniques for optimizing the operation of the reaction system. For instance, in the catalytic conversion of petroleum hydrocarbon feedstock, the dry gas pre-lift technique, the mixed temperature controlling technique by changing the feedstock injection point and injection media, the fast separation technique at reactor outlet zone, and the rapid-cooling termination technique all exert remarkable impacts on the reaction outcome and have been well practiced in the industry. Improvements in and controlling of catalyst performances during the reaction are also critically important.
Since 1970s, fluidized-bed reactors have been replaced with riser reactors for catalytic cracking reaction of petroleum hydrocarbons. Catalytic cracking reaction of petroleum hydrocarbon feedstock is a gas- and solid-phase reaction, the feedstock for the reaction is generally liquid feed which needs to absorb heat to be vaporized first, and then the gaseous reactants enter the micro-pores and channels in the catalysts to undergo catalytic reaction. The heat of vaporization for the gaseous reactants is provided by regenerated catalyst. Catalytic reactions of liquid feedstock in tubular reactors all have two stages, i.e. vaporization of liquid and gas-phase reaction. The vaporization is achieved by the heat provided by contact with the catalyst, and there is more or less reaction occurring during the vaporization. The intrinsic advantage of tubular reactors lies in the presence of a “gradient”, by which the purpose of increasing the “driving force” of the reaction can be achieved and the efficiency and selectivity of the reaction can be improved. And this is the major reason why fluidized-bed reactors are replaced with riser reactors. Petroleum hydrocarbon feedstock has complex compositions, and the feedstock entering the inlet of the reactor has a molecular weight of about 300, while the product exiting the outlet of the reactor has a molecular weight of about 100 or less. The riser reactor exhibits various reactant properties and reaction processes at different positions inside the reactor. For example, at the initial stage in the gas-phase reaction zone above the atomizing nozzle, vaporized feedstock oil mainly undergoes cracking reaction of large molecules in the feedstock oil which generally lasts for around 1.5 seconds, while in the gas-phase reaction zone at the next half of the riser, further reaction as well as isomerization and hydrogen transfer among small molecule components such as diesel, gasoline and liquefied gas, is dominant. Thus, the entire gas-phase reaction zone is often divided into “cracking reaction zone”, “reformation reaction zone”, etc. For the liquid vaporization reaction, both the temperature difference between the catalyst and the feedstock and the high temperature of the catalyst can enhance thermal cracking reaction, increase the fraction of byproducts of C2 or lower C number fractions, and affect the economic benefit. The temperatures of the gas-phase cracking reaction zone and the reformation reaction zone, the catalyst-to-oil ratio, and the catalyst activity all have significant influence on the reaction results of the entire reaction zone.
For example, with regard to the catalytic conversion of petroleum hydrocarbon-based feedstock, it has been confirmed by worldwide studies that a high temperature of the regenerated catalyst results in a low catalyst-to-oil ratio in the reaction, and a large temperature difference between the catalyst and the feedstock upon contact with the catalyst leads to a high production of dry gas and severely affects the yield; in the riser, with the proceeding of reaction, the catalyst activity rapidly decreases, and the efficiency in the reaction zone downstream of the riser is lowered, therefore affecting the outcome of the reaction. Lowering the temperature of the regenerated catalyst, increasing the catalyst-to-oil ratio, and increasing the catalyst activity in the riser have been the objectives pursued for many years for catalytic cracking units.
There are many technical means to reduce the temperature difference between the regenerated catalyst and the feedstock oil in the contact zone where the reaction feedstock liquid phase is vaporized. In order to lower the temperature upon initial contact with feedstock oil, the most straightforward way is to “transfer the low-temperature catalyst from the external catalyst cooler to the pre-lift section of the riser”. In 1990s, UOP proposed a method of transferring the low-temperature catalyst from the external catalyst cooler to the pre-lift section of the riser, in the U.S. Pat. No. 5,800,697. There are a number of related patent documents, such as U.S. Pat. No. 6,059,958, 6,558,530B1, CN01119805.1, CN1664074A, CN101191067A, and CN101191072A, etc.
U.S. Pat. No. 5,800,697 discloses a method for catalytic conversion reaction and regeneration, wherein a catalyst cooling zone is arranged beside a regenerator, the hot regenerated catalyst from a dense bed enters the cooling zone through an outlet and is cooled to an appropriate temperature by heat exchange, and then enters the bottom part of the riser reactor via a regenerated-catalyst standpipe and a slide valve to participate in the reaction, so that the catalyst-to-oil ratio of the reaction becomes an independent variable. CN101161786A discloses a conversion method of petroleum hydrocarbons, wherein the hot regenerated catalyst is cooled by a cooler and returns to the bottom part of the reactor where it goes into contact with the feedstock oil to undergo the cracking reaction; the spent catalyst is transferred to a regenerator after steam stripping to undergo coke-burning regeneration, and then is recycled or partially directed to a mixer at the bottom of the reactor. CN101191067A discloses a temperature regulating device for regenerated catalyst in a catalytic cracking apparatus, wherein a catalyst cooler is installed beside the dense bed of the regenerator, the heat exchange tubes are provided inside the catalyst cooler, a flue gas returning pipe is provided above the cooler, fluidizing rings are provided in the sections along the vertical direction of the catalyst cooler, and the cooled regenerated catalyst enters the pre-lift section of the riser reactor to participate in the reaction. CN101191071A also discloses a temperature regulating device for regenerated catalyst in a catalytic cracking apparatus, wherein a partition is provided in the dense bed of the regenerator to divide the dense bed of the regenerator into two zones, one for coke-burning regeneration and the other for catalyst cooling, a catalyst inlet of the cooler is provided at the lower part of the partition, a heat exchange tube is provided in the catalyst cooling zone, and the cooled regenerated catalyst enters the pre-lift section of the riser reactor to participate in the reaction.
Another way to reduce the temperature difference between the feedstock oil and the regenerated catalyst upon their initial contact is to elevate the temperature of the feedstock oil. CN101144028A discloses a method for cracking of hydrocarbon oil, wherein hydrocarbon oil and regenerated catalyst are heated in a heat exchanger, and then the heat-exchanged hydrocarbon oil and the heat-exchanged regenerated catalyst react upon contact with each other in the reactor.
Catalytic cracking reaction of petroleum hydrocarbons generally employs tubular reactors. However, different reaction processes take place at different positions of a riser reactor, among which vaporization of feedstock oil is carried out first, followed by catalytic cracking reaction which converts large molecule feedstock into products mainly consisting of gasoline and diesel, accompanied subsequently by further reaction of components of gasoline and diesel. Reactions at different positions require different reaction conditions. The condition to be controlled in current catalytic cracking reaction of petroleum hydrocarbon feedstock is the temperature at the outlet, which is however very different from the conditions at various portions of the riser. The condition at the outlet cannot reflect the reaction processes and results at the various portions. Controlling techniques for optimizing reaction conditions have been developed, mainly focusing on optimization of the reaction feedstock and flow scheme and optimization of reaction temperature and time duration, but little on the effect of catalyst during the reaction. The above patent documents only involve lowering the temperature of the regenerated catalyst in the pre-lift section.
In the prior art described above that lowers the temperature of the regenerated catalyst, the catalyst from the external catalyst cooler carries some oxygen and regeneration flue gas, wherein the oxygen enters the reactor and reacts with the reaction media, and poses the problem of affecting the products, and whereas the flue gas carry-over from the regenerated catalyst would increase the load of the rich gas compressor and energy consumption.
In addition, catalyst influences the reaction process not only by the catalyst temperature in the pre-life section, the temperature difference upon contact in the liquid phase (feedstock oil) vaporization zone and the catalyst-to-oil ratio before the reaction, but also considerably by the catalyst activity and the catalyst-to-oil ratio in the gas-phase cracking reaction zone after vaporization of the feedstock oil.
In addition to improvements on the catalyst in the liquid-phase vaporization zone, improvements in the catalyst effect during the reaction should also include improvements on the catalyst in the gas-phase reaction zone after vaporization of the feedstock oil, controlling of the status of the catalyst provided, and controlling of the gas carried by the catalyst.
In summary, the way the catalyst enters a reactor greatly influences the improvement in the reaction results, and it is more significant to optimize regeneration and reaction and to build a synergistic, integrated reaction-regeneration system.